The plant pathogenic bacterium Xylella fastidiosa occurs widely and often asymptomatically, yet it is associated with a few seriously bad disease outbreaks, the most recent affecting olive trees in southern Italy and adjoining regions. It also is the causal agent of Pierce’s Disease of grapevine, citrus variegated chlorosis (CVC) and coffee leaf scorch. Sicard et al. review all aspects of the interaction between Xylella fastidiosa and its diverse hosts, as well as the insect vectors responsible for its spread, with a focus on understanding why in some cases this relatively benign bacterium becomes a dangerous pest. One key take-home message is that as landscapes shift from natural, diverse environments to less diverse agricultural environments, the bacterium shifts from a harmless generalist to a destructive specialist. (Summary by Mary Williams) Annu. Rev. Phytopathol. 10.1146/annurev-phyto-080417-045849